Dot insertion



Sept. 6, 1932- E. K. SANDER/IAN ET AL 1,875,935

DOT INSERTION Filed Feb. 2s, 1931 2 sheets-sheet 1 ATTORNEY Sept. 6, 1932. E'. K. SANDEMAN ET AL DOT INSERTION Filed Feb. 26, 1951 FIGB HPF DM Ps FM- A+E I): +P@ LPF so 2 Sheets-Sheet 2 TORS lNVEN y EDWARD K.SANDEMAN LAUNCELOT E.CURRAH ATTORNEY squared.

Patented Sept. 6, 1932 UNITED STATES EDVJ'ARD K. SANDEMAN AND LANCELOT' raisers EDGAR CURRAH, 0F ALDWYCI-I, LONDON,

ENGLAND, ASSIGNORS T0 INTERNATIONAL STANDARD ELECTRIC CORPORATION, OF NEW YORK, N. Y., A CORPORATION OF DELAWARE v DOT INSERTION Application ined February 2e, 1931, serial No.

This invention relates to electric telegraphy and more particularly to an improvedmethod of dot insertion.

It is known to use, at the receiving station, a mechanical relay which produces dots continually when no signal is being received and which stays in one of its stable positions when and as long as a signal impulse is being received. This results in the dots, which either arrive greatly attentuated or are not transmitted at all, being filled in when necessary. By a dot is meant the shortest length of signal occurring.

lVith the advent of high speed telegraphy and a consequently higher dot frequency it becomes increasingly difficult to make a mechanical relay which will respond to the frequency of the dots and furthermore it'is undesirable to transmit the dot frequency because of the frequency range required.

It is an object of this invention to provide a method of dot insertion in which the above defects are largely avoided.

It is a feature of this invention that dot insertion is accomplished by means of mechanically static apparatus. By this is meant apparatus which has no moving parts and `is consequently free from the undesirable effects of inertia and momentum.

It is a further feature of the invention that thermionic valves are employed for the purpose of inserting the dots in a received signal.

According to another feature of 'the' invention, by means of an apparatus termed a squaring device the dots are inserted andv both the dots and the signal impulses are @ther features will be apparent from the following description. Y i

The invention will now be described with reference to the accompanying drawings of which Fig. 1 is a diagram illustrating the principle ofdot insertion.

Fig.` 2 shows the circuit arrangement of one form of the invention.

Fig. 3 illustrates anothermethod of carrying out the invention.

Fig. 4 illustrates a squaring device suitable for use in connection with the invention.

Two embodiments of the invention'will be described by way of example.

518,370, `and in Great Britain April 25, 1930.

Referring now to Fig. 1, time is plotted horizontally and amplitude (after magnification). is plotted vertically. At (P) is shown the type of current which would occur vwith! a code in which, in each of a number of successive contiguous intervals ofi-time, current is either applied or not applied to the line. These periods of time are numbered 1, 2, 8, 4, 5 etc. The amplitude of the current pulse is shown as +A and it is evident that, by adding to this current a steadyv direct current of amplitude 1/2Afa current of the form shown at (Q) will be obtained. This represents a typical current or voltage supplied at the transmitting end of the line'.l It is possible, of course, that this may be modified considerably by the apparatus at the transmitting end as well as by the attenuation and phasedistortion of the cable. Ow-

ing to the fact that the cableV cannot transmit the dot frequency, the current'pulses shown at (Q) will be suppressed during the time intervals 1, 2, 3, 4, 5, 6 and 14 and 15. Owing to the distortion of the, cable the current pulses at the intervals 7 8, 9, 10, 11, 12 and 13 will arrive at the receiving end in some such form as isshown at (R).l By suitable shaping means thesesignals can be restored to 4their original form Vas shown at l(S) and it is evidently necessary in order to obtain the original signal as shown at (Q) that the dots be added in the intervals 1, 2, 3, 4, 5, 6` and 14 and 15. i

It is assumed that some means of synchro-v nism lbetween the transmitting and receiving ends is available. Forsimplicity and by wayn of example, it will be'considered thati'syn-` chronism is obtained by transmitting a very low frequency wave on a carrier above the telegraph range, this low frequencywave being a sub-multiple of the ldot frequency.4 By frequency multiplication it is therefore possible to obtain a current wave of the same frequency as andv in exact synchronism with ...5 9

the dot frequency. Thel dot frequency [is here assumed tobe an alternating wave at the receiving endwhose half period is exactly equal to one of the time intervalsl', 2, 3,4,75, 6 etc. of Fig. 1. v

Referring to Fig. 2 which represents the V will give rise toa wave as at lreceiving end of a transmission system, such, for example, as a submarine cable,

valve distributor. supplying any requiredl number of channels. The high pass filter (HPF) selects the carrier wave and supp-lies itto a demodulator (DM). The output of the de'modulator (DM) supplies a synchronizing wave to vthe variable phase shifter (PS) the output of the phase yshifter feeds into the frequency multiplier (FM) ...which supplies the` dot frequency to the squaring device (S'D2). This squaring device (SD2) supplies the dot frequency havingthe form as at.(T) in Fig. ltofthe push pull amplifier (PPA) which: in turn supplies the dot`fre' quency to the receiving apparatus (R) in parallelwith signals supplied from the cable throughthe separating amplifier (SA). The

3.9-, paralyzing detector (PD) when it receives any signairfrom the cable paralyzes the ampli-iier (PPA) and prevents the dot frequency fromibeing supplied to The low pass iil'ter (LPF) selects telegraph frequencies and rejects the wave carrying the synchro-v nizing frequency. Y l

Except for thedetailed operation of the piecesofapparatus (PD) and (PPA) which is explained immediately below, the operation of the circuit is'no-w clear and it is evidentthat a4 wave received in the form onsidering now thenparalyzing device (PD), telegraph signals arriving from the cable are applied to the resistance (1) whose mid-A Y point is tapped through a-suitable bias battery (2) to the Vcommon point of the' lilaments of valves andY (4). In this way any input voltage arriving at terminals is applied differentially to the grids of the valves and sotiiatfwhen one grid is increased positively by'a certain amount the other is increasednegatively by anequal The battery is arranged to supply current to the plates of both the valves (3) and (4)]in parallel, through the resistance (6). rlhe battery (2) is arrangedto bias both Vvalves and (4)- so thatinormally in the absence of signals received from 'the cable substantially g no current flows through the resistance The bias isalso so arranged that onthev application of voltages across the terminals (13) of the'v magnitude o f those receivedfrom(SD1) an appreciable current The receiving apparatus,

flows through the resistance (6). It will be noted that the direction of' this current through the resistance (6) is quite independent of the polarity'of the voltages supplied to terminals (13) since such voltages always increase the negative bias on one vvalve (so havingpno effect on the total current since this is already substantially Zero) and decrease the negative bias on the other valve allowing a-cur'rent of the necessary amplitude to flow'. This current flowing 'through resistance (6) causes a potential vdrop to occur between its ends. It will be noted that resistance (6) is Vsituated. in the grid circuit of amplifier (PPA).

V- The midpoint of input resistance (7) is connected to one end of resistance (6) and the other end of resistance (6) is vconnected to battery (8) which is arranged to bias the tential across resistance (6) is Zero. Battery (11) supplies plate current to the plates of valves(9) and (10) through resistance (12). rlheplates of the two `valves are directly connected to the receiver in thefmanner shown.l It is evidentfromV the above that (PPA) operates as a -normal push pull amplifier. Further any changes in gridV bias due to variations in current through resistance (6) ,do not causeany variations in po tential between the ends of resistance (i2) sof that when the amplifier (PPA) is paralyzed byr yparalyzing detector (PD) no effect appears in the' output other than the `cessation ef the dot signals.

VAccording to another embodiment of the invention' the circuit arrangementv of F ig. 3 is employed. This will be lseen to comprise amplifiers, equalizers, phase compensators,

separating filters, demodulatonphase shiftampliiier for normal operation when the po- It will be seen that the connections The grid Ycircuit Y (17 voltage source (26), part of'resistance (2 4) extending from contact to source (4) and thence to cathode 12')l Valve (2) has a similar grid circuit symmetrical with respect to that of valve (1) and extending from gridfr(21) .through resistance (27),

source (16), Contact (15), part of resistance ('14) andfsource4 to cathodeV (22).

Consider now that initially there is a rela# tively large' platel current fiowging' through `re,

sistance 14)l vand a relativelyr small current through rresistance (24) Assume that sources (16) and (26) were initially adjusted so that in the absence of plate current through either valve the potentials on the grids would be equal to the value required to make the valves operate as normal ampliers. In this condition with plate current through resistance (14), grid (21) is made negative with regard to its normal operating condition as an amplifier by an amount equal to the voltage drop in that part of resistance (14) ,included between (15) and (4) If this drop in potential of grid (21) is sufficient to effectively prevent any plate current flowing through resistance (24), then grid (11) is biased in its normal.

along resistance (24) between battery (4) andY tapping point (25). In this way valve (1) is held in a stable condition with no plate current and valve (2) in a stable condition with normal plate current, the exact reverse of the stable condition obtained before the impulse was applied to grid (17). Application of a voltage to resistance (27) such as to make grid (21) suiiiciently negative for a short period of time would set up the first condition again. It is evident that the amount of negative bias applied to each grid by plate current flowing through the other valve depends on the position of the tappings and therefore the posi tion of the tappings controls the magnitude of the received impulse which must arrive before the opposite condition can be set up.

Referring now to Fig. 3, it will be seenthat the squaring device will be kept continually supplied with dots from the frequency multiplier be applied to the squaring device across resistance (17).

In the absence of signals the dots will be squared and passed to the receiver On arrival of signals which may be applied (in the correct phase) to resistance (27) the squaring device will remain in one of its stable positions while the signal persists, the dots being therefore suppressed. It is of course necessary to ensure that the amplitude of the signal voltage applied to the squaring device is suiiiciently large, compa-red with the dot voltage, that the latter is unable to operate the device while the signal persists. In other words the squaring device should be effectively paralyzed by the signal. Although the amplitude of the input signal is made larger than that of the dot input, the amplitudes will be Y (FM) and the voltage of the dots mayV the same in the output (which may be taken across resistance 24) because the squaring device always flops77 over to a constant limiting value.

What is claimed is:

1. A telegraph system comprising a receiving device, mechanically static apparatus adapted to supply dot impulses, a signal squaring device to which is applied the received signal and said dot impulses, and wherein said squaring device is adapted to supply said dot impulses to the receiving device during non-signalling periods and shaped signals complete with inserted dot impulses during signalling periods.

2. A telegraph system comprising a transmitting station, a receiving station, means for synchronizing the operation ofdevices at said stations comprisingV a current of-predetermined frequency, mechanically static apparatus at said receiving station adapted to supply dot impulses to a receiving device and means whereby the said dot impulses are derived from the synchronizing current.

3. A telegraph system comprising a receiving device, mechanically static means forV supplying continuously dot impulses to said device `during non-signaling periods and means adapted to shape the receivedv signals upon their arrival andbefore they are impressed on said receiving device.

4. A telegraph system comprising a thermionic valve device adapted to supply continuously to the receiver dot impulses 'derived from a synchronizing current and means for shaping said dot impulses before they are impressed on said receiver.

5. A telegraph system comprising a mechanically static apparatusadapted to supply continuously to the receiver dot impulses derived from asynchronizing current, means operated by an incoming signal to paralyze said mechanically static apparatus to suppress the supply of dots and a signal shaping device adapted to shape said dot impulses and said signals before they are impressed on l said receiver.

In witness whereof we hereunto subscribe our names this 28 day of January, 1931.

EDWARD K. SANDEMAN. LAUNOELOT EDGAR CURRAH. 

